This invention relates to pumps in general and more specifically to a cryogenic pump for pumping a liquid at or below the boiling point.
Various types of pumps and pumping apparatus are well-known in the art and have been used for decades, and in many cases for centuries, to pump any of a wide variety of materials. One common type of pump is the centrifugal pump, so named because it pumps the material by centrifugal action, i.e., by using a spinning impeller to accelerate the material being pumped radially outward into a surrounding casing or chamber. Centrifugal pumps come in a wide range of sizes and configurations and may be used in a wide range of applications. For example, centrifugal pumps are commonly used to pump water and other liquids. Centrifugal pumps may be used to pump gases as well and are commonly used in superchargers and in turbo-superchargers for internal combustion engines to compress or pump the intake charge (e.g, air or a fuel/air mix) into the engine. Centrifugal pumps or compressors have also been used in jet engines.
Centrifugal pumps typically comprise an impeller that is mounted for rotation within a casing or chamber. The impeller usually comprises a round or circular member having a central axis or hub about which the impeller rotates. The impeller is also provided with one or more blades or vanes which extend generally radially outward from the hub to the outer circumference of the impeller. The pump inlet is provided at or near the hub or inner radius of the impeller. The outlet is usually provided at one or more locations in the chamber or casing that surrounds the outer circumference of the impeller. Once the impeller is set in motion (i.e., rotated) the material being pumped is accelerated radially outward by the spinning vanes or blades on the impeller. The velocity imparted to the material is converted into pressure in the casing that surrounds the impeller and is commonly referred to as the diffuser section or simply, the diffuser. The pressurized material is then drawn-off through the one or more pump outlets provided. The material required to replace the material accelerated by the impeller is drawn into the pump inlet near the hub of the impeller.
It is well-known to provide the impeller vanes or blades with different shapes depending on the type of material being pumped and on the performance parameters (e.g., pressure ratio, discharge velocity, pumping stability, etc.) desired for the particular application. Many pumps are provided with radially oriented blades or vanes, as they tend to be the easiest to manufacture. However, other blade configurations may be better for certain applications. For example, centrifugal compressors have been produced with forward curved blades (i.e., blades that are curved in the direction of rotation of the impeller) and backward curved blades (i.e., blades that are curved in the direction opposite the direction of rotation of the impeller). Generally speaking, forward curved blades provide a greater pressure ratio or xe2x80x9cheadxe2x80x9d for a given volume flow rate (at constant impeller rpm), with radial blades and backward curved blades providing progressively lower pressure ratios at the same volume flow rate. However, other considerations associated with the particular application may dictate whether the best arrangement is to have forward, backward, or radial impeller blades or vanes.
It is also generally desired to vary the cross-sectional areas defined by the flow passages created between adjacent blades or vanes on the impeller. For example, in certain applications it may be desirable for the cross-sectional area of a given flow passage to decrease with radial distance from the hub. In other applications, it may be desirable for the cross-sectional area to remain constant, or even increase, with increasing radial distance. Impellers having such cross-sectional area variations have been designed and are being used, but typically involve complex shapes which can only be formed by casting processes.
While centrifugal pumps of the type described above work well and are being used, they still are not without their disadvantages. For example, the impellers used in such pumps are typically formed by casting and may need to be subsequently machined depending the blade shape required. Furthermore, while it is possible to provide the flow channels defined by the blades or vanes with varying cross-sectional areas as a function of radial distance from the hub, impellers having such characteristics typically have complex shapes that are difficult and expensive to manufacture.
Another problem that currently exists in the field of material pumping relates to the pumping of liquids and other materials that are maintained at temperatures that are at or near their boiling points. The liquids involved in such applications are typically cryogenic liquids, such as liquid nitrogen, liquid oxygen, and others, although there are also occasions wherein the liquids are not cryogenic. The pumping of a liquid that is at or near its boiling point is difficult because the reduced pressures located in the inlet portions of the pump can cause the liquid to boil, resulting in pump cavitation and a loss of pumping efficiency. Such boiling problems are often made worse if the pump body is warmer than the liquid being pumped. In such cases, heat from the pump body and other components is transferred to the liquid being pumped. The extra heat is often sufficient to boil the liquid which, again, can lead to cavitation and a loss of pumping efficiency.
Consequently, a need remains for a pump having an impeller that can be quickly and easily manufactured while at the same time allowing the flow passages defined by the impeller to be formed with any of a wide range of cross-sectional areas and configurations in order to optimize pump operation. Other advantages could be realized if a pump having such an improved impeller design could be used to pump liquids at or near their boiling points, but with a reduced likelihood that the liquid will vaporize and/or boil as it is being pumped.
A pump according to the present invention may comprise a housing having an inlet and an outlet. An impeller assembly mounted for rotation within the housing includes a first impeller piece having a first mating surface thereon and a second impeller piece having a second mating surface therein. The second mating surface of the second impeller piece includes at least one groove therein so that at least one flow channel is defined between the groove and the first mating surface of the first impeller piece. A drive system operatively associated with the impeller assembly rotates the impeller assembly within the housing.